Vertical thin film transistors with surround gates

ABSTRACT

A method is provided that includes forming a transistor by forming a first a rail gate disposed in a first direction above a substrate, forming a second rail gate disposed in a second direction above the substrate, the second direction perpendicular to the first direction, and forming a bridge section disposed between the first rail gate and the second rail gate.

BACKGROUND

Semiconductor memory is widely used in various electronic devices suchas mobile computing devices, mobile phones, solid-state drives, digitalcameras, personal digital assistants, medical electronics, servers, andnon-mobile computing devices. Semiconductor memory may includenon-volatile memory or volatile memory. A non-volatile memory deviceallows information to be stored or retained even when the non-volatilememory device is not connected to a source of power (e.g., a battery).

Examples of non-volatile memory include flash memory (e.g., NAND-typeand NOR-type flash memory), Electrically Erasable Programmable Read-OnlyMemory (EEPROM), ferroelectric memory (e.g., FeRAM), magnetoresistivememory (e.g., MRAM), and phase change memory (e.g., PRAM). In recentyears, non-volatile memory devices have been scaled in order to reducethe cost per bit. However, as process geometries shrink, many design andprocess challenges are presented. These challenges include increasedvariability in memory cell I-V characteristics, reduced memory cellsensing currents, and increased bit line settling times.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts an embodiment of a memory system and a host.

FIG. 1B depicts an embodiment of memory core control circuits.

FIG. 1C depicts an embodiment of a memory core.

FIG. 1D depicts an embodiment of a memory bay.

FIG. 1E depicts an embodiment of a memory block.

FIG. 1F depicts another embodiment of a memory bay.

FIG. 2A depicts a schematic diagram of the memory bay of FIG. 1F.

FIG. 2B depicts a schematic diagram of a memory bay arrangement whereinword lines and bit lines are shared across memory blocks, and both rowdecoders and column decoders are split.

FIG. 3A depicts an embodiment of a portion of a monolithicthree-dimensional memory array.

FIG. 3B depicts a subset of the memory array and routing layers of anembodiment of a three-dimensional memory array.

FIG. 4A depicts an embodiment of a portion of a monolithicthree-dimensional memory array.

FIG. 4B depicts an embodiment of a portion of a monolithicthree-dimensional memory array that includes vertical strips of anon-volatile memory material.

FIGS. 5A-5D depict various views of an embodiment monolithicthree-dimensional memory array.

FIGS. 6A1-6L2 are cross-sectional views of a portion of a substrateduring an example fabrication of the monolithic three-dimensional memoryarray of FIGS. 5A-5C.

DETAILED DESCRIPTION

Technology is described for monolithic three-dimensional memory arrays.In particular, a monolithic three-dimensional memory array is formedthat includes a vertically-oriented bit line select transistor. A methodis provided for forming the vertically-oriented bit line selecttransistor by forming a first a rail gate disposed in a first directionabove a substrate, forming a second rail gate disposed in a seconddirection above the substrate, the second direction perpendicular to thefirst direction, and forming a bridge section disposed between the firstrail gate and the second rail gate

In some embodiments, a memory array may include a cross-point memoryarray. A cross-point memory array may refer to a memory array in whichtwo-terminal memory cells are placed at the intersections of a first setof control lines (e.g., word lines) arranged in a first direction and asecond set of control lines (e.g., bit lines) arranged in a seconddirection perpendicular to the first direction. The two-terminal memorycells may include a resistance-switching material, such as a phasechange material, a ferroelectric material, or a metal oxide (e.g.,nickel oxide or hafnium oxide). In some cases, each memory cell in across-point memory array may be placed in series with a steering elementor an isolation element, such as a diode, to reduce leakage currents. Incross-point memory arrays where the memory cells do not include anisolation element, controlling and minimizing leakage currents may be asignificant issue, especially because leakage currents may vary greatlyover biasing voltage and temperature.

In one embodiment, a non-volatile storage system may include one or moretwo-dimensional arrays of non-volatile memory cells. The memory cellswithin a two-dimensional memory array may form a single layer of memorycells and may be selected via control lines (e.g., word lines and bitlines) in the X and Y directions. In another embodiment, a non-volatilestorage system may include one or more monolithic three-dimensionalmemory arrays in which two or more layers of memory cells may be formedabove a single substrate without any intervening substrates. In somecases, a three-dimensional memory array may include one or more verticalcolumns of memory cells located above and orthogonal to a substrate. Inone example, a non-volatile storage system may include a memory arraywith vertical bit lines or bit lines that are arranged orthogonal to asemiconductor substrate. The substrate may include a silicon substrate.The memory array may include rewriteable non-volatile memory cells,wherein each memory cell includes a reversible resistance-switchingelement without an isolation element in series with the reversibleresistance-switching element (e.g., no diode in series with thereversible resistance-switching element).

In some embodiments, a non-volatile storage system may include anon-volatile memory that is monolithically formed in one or morephysical levels of arrays of memory cells having an active area disposedabove a silicon substrate. The non-volatile storage system may alsoinclude circuitry associated with the operation of the memory cells(e.g., decoders, state machines, page registers, or control circuitryfor controlling the reading or programming of the memory cells). Thecircuitry associated with the operation of the memory cells may belocated above the substrate or located within the substrate.

In some embodiments, a non-volatile storage system may include amonolithic three-dimensional memory array. The monolithicthree-dimensional memory array may include one or more levels of memorycells. Each memory cell within a first level of the one or more levelsof memory cells may include an active area that is located above asubstrate (e.g., above a single-crystal substrate or a crystallinesilicon substrate). In one example, the active area may include asemiconductor junction (e.g., a P-N junction). The active area mayinclude a portion of a source or drain region of a transistor. Inanother example, the active area may include a channel region of atransistor.

FIG. 1A depicts one embodiment of a memory system 100 and a host 102.Memory system 100 may include a non-volatile storage system interfacingwith host 102 (e.g., a mobile computing device). In some cases, memorysystem 100 may be embedded within host 102. In other cases, memorysystem 100 may include a memory card. As depicted, memory system 100includes a memory chip controller 104 and a memory chip 106. Although asingle memory chip 106 is depicted, memory system 100 may include morethan one memory chip (e.g., four, eight or some other number of memorychips). Memory chip controller 104 may receive data and commands fromhost 102 and provide memory chip data to host 102.

Memory chip controller 104 may include one or more state machines, pageregisters, SRAM, and control circuitry for controlling the operation ofmemory chip 106. The one or more state machines, page registers, SRAM,and control circuitry for controlling the operation of memory chip 106may be referred to as managing or control circuits. The managing orcontrol circuits may facilitate one or more memory array operations,such as forming, erasing, programming, or reading operations.

In some embodiments, the managing or control circuits (or a portion ofthe managing or control circuits) for facilitating one or more memoryarray operations may be integrated within memory chip 106. Memory chipcontroller 104 and memory chip 106 may be arranged on a singleintegrated circuit. In other embodiments, memory chip controller 104 andmemory chip 106 may be arranged on different integrated circuits. Insome cases, memory chip controller 104 and memory chip 106 may beintegrated on a system board, logic board, or a PCB.

Memory chip 106 includes memory core control circuits 108 and a memorycore 110. Memory core control circuits 108 may include logic forcontrolling the selection of memory blocks (or arrays) within memorycore 110, controlling the generation of voltage references for biasing aparticular memory array into a read or write state, or generating rowand column addresses.

Memory core 110 may include one or more two-dimensional arrays of memorycells or one or more three-dimensional arrays of memory cells. In oneembodiment, memory core control circuits 108 and memory core 110 arearranged on a single integrated circuit. In other embodiments, memorycore control circuits 108 (or a portion of memory core control circuits108) and memory core 110 may be arranged on different integratedcircuits.

A memory operation may be initiated when host 102 sends instructions tomemory chip controller 104 indicating that host 102 would like to readdata from memory system 100 or write data to memory system 100. In theevent of a write (or programming) operation, host 102 will send tomemory chip controller 104 both a write command and the data to bewritten. The data to be written may be buffered by memory chipcontroller 104 and error correcting code (ECC) data may be generatedcorresponding with the data to be written. The ECC data, which allowsdata errors that occur during transmission or storage to be detectedand/or corrected, may be written to memory core 110 or stored innon-volatile memory within memory chip controller 104. In oneembodiment, the ECC data are generated and data errors are corrected bycircuitry within memory chip controller 104.

Memory chip controller 104 controls operation of memory chip 106. In oneexample, before issuing a write operation to memory chip 106, memorychip controller 104 may check a status register to make sure that memorychip 106 is able to accept the data to be written. In another example,before issuing a read operation to memory chip 106, memory chipcontroller 104 may pre-read overhead information associated with thedata to be read. The overhead information may include ECC dataassociated with the data to be read or a redirection pointer to a newmemory location within memory chip 106 in which to read the datarequested. Once a read or write operation is initiated by memory chipcontroller 104, memory core control circuits 108 may generate theappropriate bias voltages for word lines and bit lines within memorycore 110, and generate the appropriate memory block, row, and columnaddresses.

In some embodiments, one or more managing or control circuits may beused for controlling the operation of a memory array. The one or moremanaging or control circuits may provide control signals to a memoryarray to perform a read operation and/or a write operation on the memoryarray. In one example, the one or more managing or control circuits mayinclude any one of or a combination of control circuitry, state machine,decoders, sense amplifiers, read/write circuits, and/or controllers. Theone or more managing circuits may perform or facilitate one or morememory array operations including erasing, programming, or readingoperations. In one example, one or more managing circuits may include anon-chip memory controller for determining row and column address, wordline and bit line addresses, memory array enable signals, and datalatching signals.

FIG. 1B depicts an embodiment of memory core control circuits 108. Asdepicted, memory core control circuits 108 include address decoders 120,voltage generators for selected control lines 122, voltage generatorsfor unselected control lines 124 and signal generators for referencesignals 126 (described in more detail below). Control lines may includeword lines, bit lines, or a combination of word lines and bit lines.Selected control lines may include selected word lines and/or selectedbit lines that are used to place memory cells into a selected state.Unselected control lines may include unselected word lines and/orunselected bit lines that are used to place memory cells into anunselected state.

Address decoders 120 may generate memory block addresses, as well as rowaddresses and column addresses for a particular memory block. Voltagegenerators (or voltage regulators) for selected control lines 122 mayinclude one or more voltage generators for generating selected controlline voltages. Voltage generators for unselected control lines 124 mayinclude one or more voltage generators for generating unselected controlline voltages. Signal generators for reference signals 126 may includeone or more voltage and/or current generators for generating referencevoltage and/or current signals.

FIGS. 1C-1F depict one embodiment of a memory core organization thatincludes a memory core having multiple memory bays, and each memory bayhaving multiple memory blocks. Although a memory core organization isdisclosed where memory bays include memory blocks, and memory blocksinclude a group of memory cells, other organizations or groupings alsocan be used with the technology described herein.

FIG. 1C depicts one embodiment of memory core 110 in FIG. 1A. Asdepicted, memory core 110 includes memory bay 130 and memory bay 132. Insome embodiments, the number of memory bays per memory core can bedifferent for different implementations. For example, a memory core mayinclude only a single memory bay or multiple memory bays (e.g., 16 orother number of memory bays).

FIG. 1D depicts one embodiment of memory bay 130 in FIG. 1C. Asdepicted, memory bay 130 includes memory blocks 140-144 and read/writecircuits 146. In some embodiments, the number of memory blocks permemory bay may be different for different implementations. For example,a memory bay may include one or more memory blocks (e.g., 32 or othernumber of memory blocks per memory bay). Read/write circuits 146 includecircuitry for reading and writing memory cells within memory blocks140-144.

As depicted, read/write circuits 146 may be shared across multiplememory blocks within a memory bay. This allows chip area to be reducedbecause a single group of read/write circuits 146 may be used to supportmultiple memory blocks. However, in some embodiments, only a singlememory block may be electrically coupled to read/write circuits 146 at aparticular time to avoid signal conflicts.

In some embodiments, read/write circuits 146 may be used to write one ormore pages of data into memory blocks 140-144 (or into a subset of thememory blocks). The memory cells within memory blocks 140-144 may permitdirect over-writing of pages (i.e., data representing a page or aportion of a page may be written into memory blocks 140-144 withoutrequiring an erase or reset operation to be performed on the memorycells prior to writing the data).

In one example, memory system 100 in FIG. 1A may receive a write commandincluding a target address and a set of data to be written to the targetaddress. Memory system 100 may perform a read-before-write (RBW)operation to read the data currently stored at the target address and/orto acquire overhead information (e.g., ECC information) beforeperforming a write operation to write the set of data to the targetaddress.

In some cases, read/write circuits 146 may be used to program aparticular memory cell to be in one of three or more data/resistancestates (i.e., the particular memory cell may include a multi-levelmemory cell). In one example, read/write circuits 146 may apply a firstvoltage difference (e.g., 2V) across the particular memory cell toprogram the particular memory cell into a first state of the three ormore data/resistance states or a second voltage difference (e.g., 1V)across the particular memory cell that is less than the first voltagedifference to program the particular memory cell into a second state ofthe three or more data/resistance states.

Applying a smaller voltage difference across the particular memory cellmay cause the particular memory cell to be partially programmed orprogrammed at a slower rate than when applying a larger voltagedifference. In another example, read/write circuits 146 may apply afirst voltage difference across the particular memory cell for a firsttime period (e.g., 150 ns) to program the particular memory cell into afirst state of the three or more data/resistance states or apply thefirst voltage difference across the particular memory cell for a secondtime period less than the first time period (e.g., 50 ns). One or moreprogramming pulses followed by a memory cell verification phase may beused to program the particular memory cell to be in the correct state.

FIG. 1E depicts one embodiment of memory block 140 in FIG. 1D. Asdepicted, memory block 140 includes a memory array 150, row decoder 152,and column decoder 154. Memory array 150 may include a contiguous groupof memory cells having contiguous word lines and bit lines. Memory array150 may include one or more layers of memory cells. Memory array 150 mayinclude a two-dimensional memory array or a three-dimensional memoryarray.

Row decoder 152 decodes a row address and selects a particular word linein memory array 150 when appropriate (e.g., when reading or writingmemory cells in memory array 150). Column decoder 154 decodes a columnaddress and selects one or more bit lines in memory array 150 to beelectrically coupled to read/write circuits, such as read/write circuits146 in FIG. 1D. In one embodiment, the number of word lines is 4K permemory layer, the number of bit lines is 1K per memory layer, and thenumber of memory layers is 4, providing a memory array 150 containing 16L memory cells.

FIG. 1F depicts one embodiment of a memory bay 134. Memory bay 134 isone example of an alternative implementation for memory bay 130 in FIG.1D. In some embodiments, row decoders, column decoders, and read/writecircuits may be split or shared between memory arrays. As depicted, rowdecoder 152 b is shared between memory arrays 150 a and 150 b becauserow decoder 152 b controls word lines in both memory arrays 150 a and150 b (i.e., the word lines driven by row decoder 152 b are shared).

Row decoders 152 a and 152 b may be split such that even word lines inmemory array 150 a are driven by row decoder 152 a and odd word lines inmemory array 150 a are driven by row decoder 152 b. Row decoders 152 cand 152 b may be split such that even word lines in memory array 150 bare driven by row decoder 152 c and odd word lines in memory array 150 bare driven by row decoder 152 b.

Column decoders 154 a and 154 b may be split such that even bit lines inmemory array 150 a are controlled by column decoder 154 b and odd bitlines in memory array 150 a are driven by column decoder 154 a. Columndecoders 154 c and 154 d may be split such that even bit lines in memoryarray 150 b are controlled by column decoder 154 d and odd bit lines inmemory array 150 b are driven by column decoder 154 c.

The selected bit lines controlled by column decoder 154 a and columndecoder 154 c may be electrically coupled to read/write circuits 146 a.The selected bit lines controlled by column decoder 154 b and columndecoder 154 d may be electrically coupled to read/write circuits 146 b.Splitting the read/write circuits into read/write circuits 146 a and 146b when the column decoders are split may allow for a more efficientlayout of the memory bay.

FIG. 2A depicts one embodiment of a schematic diagram (including wordlines and bit lines) corresponding with memory bay 134 in FIG. 1F. Asdepicted, word lines WL1, WL3, and WL5 are shared between memory arrays150 a and 150 b and controlled by row decoder 152 b of FIG. 1F. Wordlines WL0, WL2, WL4, and WL6 are driven from the left side of memoryarray 150 a and controlled by row decoder 152 a of FIG. 1F. Word linesWL14, WL16, WL18, and WL20 are driven from the right side of memoryarray 150 b and controlled by row decoder 152 c of FIG. 1F.

Bit lines BL0, BL2, BL4, and BL6 are driven from the bottom of memoryarray 150 a and controlled by column decoder 154 b of FIG. 1F. Bit linesBL1, BL3, and BL5 are driven from the top of memory array 150 a andcontrolled by column decoder 154 a of FIG. 1F. Bit lines BL7, BL9, BL11,and BL13 are driven from the bottom of memory array 150 b and controlledby column decoder 154 d of FIG. 1F. Bit lines BL8, BL10, and BL12 aredriven from the top of memory array 150 b and controlled by columndecoder 154 c of FIG. 1F.

In one embodiment, memory arrays 150 a and 150 b may include memorylayers that are oriented in a horizontal plane that is horizontal to thesupporting substrate. In another embodiment, memory arrays 150 a and 150b may include memory layers that are oriented in a vertical plane thatis vertical with respect to the supporting substrate (i.e., the verticalplane is perpendicular to the supporting substrate).

FIG. 2B depicts one embodiment of a schematic diagram (including wordlines and bit lines) corresponding with a memory bay arrangement whereinword lines and bit lines are shared across memory blocks, and both rowdecoders and column decoders are split. Sharing word lines and/or bitlines helps to reduce layout area since a single row decoder and/orcolumn decoder can be used to support two memory arrays.

As depicted, word lines WL1, WL3, and WL5 are shared between memoryarrays 150 a and 150 b, and word lines WL8, WL10 and WL12 are sharedbetween memory arrays 150 c and 150 d. Bit lines BL1, BL3, and BL5 areshared between memory arrays 150 a and 150 c, and bit lines BL8, BL10,and BL12 are shared between memory arrays 150 b and 150 d.

Row decoders are split such that word lines WL0, WL2, WL4, and WL6 aredriven from the left side of memory array 150 a and word lines WL1, WL3,and WL5 are driven from the right side of memory array 150 a. Likewise,word lines WL7, WL9, WL11, and WL13 are driven from the left side ofmemory array 150 c and word lines WL8, WL10, and WL12 are driven fromthe right side of memory array 150 c.

Column decoders are split such that bit lines BL0, BL2, BL4, and BL6 aredriven from the bottom of memory array 150 a and bit lines BL1, BL3, andBL5 are driven from the top of memory array 150 a. Likewise, bit linesBL21, BL23, BL25, and BL27 are driven from the top of memory array 150 dand bit lines BL8, BL10, and BL12 are driven from the bottom of memoryarray 150 d. Splitting row and/or column decoders also helps to relievelayout constraints (e.g., the column decoder pitch can be relieved by 2×since the split column decoders need only drive every other bit lineinstead of every bit line).

FIG. 3A depicts one embodiment of a portion of a monolithicthree-dimensional memory array 300 that includes a first memory level302 and a second memory level 304 positioned above first memory level302. Memory array 300 is one example of an implementation for memoryarray 150 in FIG. 1E. Bit lines 306 and 308 are arranged in a firstdirection and word lines 310 are arranged in a second directionperpendicular to the first direction. As depicted, the upper conductorsof first memory level 302 may be used as the lower conductors of secondmemory level 304. In a memory array with additional layers of memorycells, there would be corresponding additional layers of bit lines andword lines.

Memory array 300 includes a plurality of memory cells 312. Memory cells312 may include re-writeable memory cells, and may include non-volatilememory cells or volatile memory cells. With respect to first memorylevel 302, a first portion of memory cells 312 are between and connectto bit lines 306 and word lines 310. With respect to second memory level304, a second portion of memory cells 312 are between and connect to bitlines 308 and word lines 310. In one embodiment, each memory cell 312includes a steering element (e.g., a diode) and a memory element (i.e.,a state change element).

In one example, the diodes of first memory level 302 may be upwardpointing diodes as indicated by arrow A₁ (e.g., with p regions at thebottom of the diodes), whereas the diodes of second memory level 304 maybe downward pointing diodes as indicated by arrow A₂ (e.g., with nregions at the bottom of the diodes), or vice versa. In anotherembodiment, each memory cell 312 includes only a state change element.The absence of a diode (or other steering element) from a memory cellmay reduce the process complexity and costs associated withmanufacturing a memory array.

In one embodiment, memory cells 312 include re-writable non-volatilememory cells including a reversible resistance-switching element. Areversible resistance-switching element may include a reversibleresistance-switching material having a resistance that may be reversiblyswitched between two or more states. In one embodiment, the reversibleresistance-switching material may include a metal oxide (e.g., a binarymetal oxide). The metal oxide may include nickel oxide, hafnium oxide,or some other metal oxide material. In another embodiment, thereversible resistance-switching material may include a phase changematerial. The phase change material may include a chalcogenide material.In some cases, the re-writeable non-volatile memory cells may includeresistive RAM (ReRAM) devices.

In another embodiment, memory cells 312 may include conductive bridgememory elements. A conductive bridge memory element also may be referredto as a programmable metallization cell. A conductive bridge memoryelement may be used as a state change element based on the physicalrelocation of ions within a solid electrolyte. In some cases, aconductive bridge memory element may include two solid metal electrodes,one relatively inert (e.g., tungsten) and the other electrochemicallyactive (e.g., silver or copper), with a thin film of the solidelectrolyte between the two electrodes. As temperature increases, themobility of the ions also increases causing the programming thresholdfor the conductive bridge memory cell to decrease. Thus, the conductivebridge memory element may have a wide range of programming thresholdsover temperature.

In one embodiment of a read operation, the data stored in one of theplurality of memory cells 312 may be read by biasing one of the wordlines (i.e., the selected word line) to a selected word line voltage inread mode (e.g., 0V). A sense amplifier may then be used to bias aselected bit line connected to the selected memory cell to the selectedbit line voltage in read mode (e.g., 1.0V). In some cases, to avoidsensing leakage current from the many unselected word lines to theselected bit line, the unselected word lines may be biased to the samevoltage as the selected bit lines (e.g., 1.0V). To avoid leakage currentfrom the selected word line to the unselected bit lines, the unselectedbit lines may be biased to the same voltage as the selected word line(e.g., 0V). However, biasing the unselected word lines to the samevoltage as the selected bit lines and biasing the unselected bit linesto the same voltage as the selected word line may place a substantialvoltage stress across the unselected memory cells driven by both theunselected word lines and the unselected bit lines.

In an alternative read biasing scheme, both the unselected word linesand the unselected bit lines may be biased to an intermediate voltagethat is between the selected word line voltage and the selected bit linevoltage. Applying the same voltage to both the unselected word lines andthe unselected bit lines may reduce the voltage stress across theunselected memory cells driven by both the unselected word lines and theunselected bit lines.

However, the reduced voltage stress comes at the expense of increasedleakage currents associated with the selected word line and the selectedbit line. Before the selected word line voltage has been applied to theselected word line, the selected bit line voltage may be applied to theselected bit line, and a sense amplifier may then sense an auto zeroamount of current through the selected memory bit line which issubtracted from the bit line current in a second current sensing whenthe selected word line voltage is applied to the selected word line. Theleakage current may be subtracted out by using the auto zero currentsensing.

In one embodiment of a write operation, the reversibleresistance-switching material may be in an initial high-resistance statethat is switchable to a low-resistance state upon application of a firstvoltage and/or current. Application of a second voltage and/or currentmay return the reversible resistance-switching material back to thehigh-resistance state. Alternatively, the reversibleresistance-switching material may be in an initial low-resistance statethat is reversibly switchable to a high-resistance state uponapplication of the appropriate voltage(s) and/or current(s).

When used in a memory cell, one resistance state may represent a binarydata “0,” and another resistance state may represent a binary data “1.”In some cases, a memory cell may be considered to include more than twodata/resistance states (i.e., a multi-level memory cell). In some cases,a write operation may be similar to a read operation except with alarger voltage range placed across the selected memory cells.

The process of switching the resistance of a reversibleresistance-switching element from a high-resistance state to alow-resistance state may be referred to as SETTING the reversibleresistance-switching element. The process of switching the resistancefrom the low-resistance state to the high-resistance state may bereferred to as RESETTING the reversible resistance-switching element.The high-resistance state may be associated with binary data “1” and thelow-resistance state may be associated with binary data “0.” In otherembodiments, SETTING and RESETTING operations and/or the data encodingcan be reversed. In some embodiments, the first time aresistance-switching element is SET may require a higher than normalprogramming voltage and may be referred to as a FORMING operation.

In one embodiment of a write operation, data may be written to one ofthe plurality of memory cells 312 by biasing one of the word lines(i.e., the selected word line) to the selected word line voltage inwrite mode (e.g., 5V). A write circuit may be used to bias the bit lineconnected to the selected memory cell to the selected bit line voltagein write mode (e.g., 0V).

In some cases, to prevent program disturb of unselected memory cellssharing the selected word line, the unselected bit lines may be biasedsuch that a first voltage difference between the selected word linevoltage and the unselected bit line voltage is less than a first disturbthreshold. To prevent program disturb of unselected memory cells sharingthe selected bit line, the unselected word lines may be biased such thata second voltage difference between the unselected word line voltage andthe selected bit line voltage is less than a second disturb threshold.The first disturb threshold and the second disturb threshold may bedifferent depending on the amount of time in which the unselected memorycells susceptible to disturb are stressed.

In one write biasing scheme, both the unselected word lines and theunselected bit lines may be biased to an intermediate voltage that isbetween the selected word line voltage and the selected bit linevoltage. The intermediate voltage may be generated such that a firstvoltage difference across unselected memory cells sharing a selectedword line is greater than a second voltage difference across otherunselected memory cells sharing a selected bit line. One reason forplacing the larger voltage difference across the unselected memory cellssharing a selected word line is that the memory cells sharing theselected word line may be verified immediately after a write operationto detect a write disturb.

FIG. 3B depicts a subset of the memory array and routing layers of oneembodiment of a three-dimensional memory array, such as memory array 150in FIG. 1E. As depicted, the Memory Array layers are positioned abovethe Substrate. The Memory Array layers include bit line layers BL0, BL1and BL2, and word line layers WL0 and WL1. In other embodiments,additional bit line and word line layers also can be implemented.Supporting circuitry (e.g., row decoders, column decoders, andread/write circuits) may be arranged on the surface of the Substratewith the Memory Array layers fabricated above the supporting circuitry.

An integrated circuit implementing a three-dimensional memory array alsomay include multiple metal layers for routing signals between differentcomponents of the supporting circuitry, and between the supportingcircuitry and the bit lines and word lines of the memory array. Theserouting layers can be arranged above the supporting circuitry that isimplemented on the surface of the Substrate and below the Memory Arraylayers.

As depicted in FIG. 3B, two metal layers R1 and R2 are used for routinglayers. However, other embodiments can include more or less than twometal layers. In one example, these metal layers R1 and R2 are formed oftungsten (about 1 ohm/square). Positioned above the Memory Array layersmay be one or more top metal layers used for routing signals betweendifferent components of the integrated circuit, such as the Top Metallayer. In one example, the Top Metal layer is formed of copper oraluminum (about 0.05 ohms/square), which may provide a smallerresistance per unit area than metal layers R1 and R2. In some cases,metal layers R1 and R2 may not be implemented using the same materialsas those used for the Top Metal layers because the metal used for R1 andR2 must be able to withstand the processing steps for fabricating theMemory Array layers on top of R1 and R2 (e.g., satisfying a particularthermal budget during fabrication).

FIG. 4A depicts one embodiment of a portion of a monolithicthree-dimensional memory array 400 that includes a first memory level410, and a second memory level 412 positioned above first memory level410. Memory array 400 is one example of an implementation for memoryarray 150 in FIG. 1E. Local bit lines LBL₁₁-LBL₃₃ are arranged in afirst direction (e.g., a z-direction) and word lines WL₁₀-WL₂₃ arearranged in a second direction (e.g., an x-direction) perpendicular tothe first direction. This arrangement of vertical bit lines in amonolithic three-dimensional memory array is one embodiment of avertical bit line memory array.

As depicted, disposed between the intersection of each local bit lineand each word line is a particular memory cell (e.g., memory cell M₁₁₁is disposed between local bit line LBL₁₁ and word line WL₁₀). Theparticular memory cell may include a floating gate device, a charge trapdevice (e.g., using a silicon nitride material), a reversibleresistance-switching device, a ReRAM device, or other similar device.The global bit lines GBL₁-GBL₃ are arranged in a third direction (e.g.,a y-direction) that is perpendicular to both the first direction and thesecond direction.

Each local bit line LBL₁₁-LBL₃₃ has an associated bit line selecttransistor Q₁₁-Q₃₃, respectively. Bit line select transistors Q₁₁-Q₃₃may be field effect transistors, such as shown, or may be any othertransistors. As depicted, bit line select transistors Q₁₁-Q₃₁ areassociated with local bit lines LBL₁₁-LBL₃₁, respectively, and may beused to connect local bit lines LBL₁₁-LBL₃₁ to global bit linesGBL₁-GBL₃, respectively, using row select line SG₁. In particular, eachof bit line select transistors Q₁₁-Q₃₁ has a first terminal (e.g., adrain/source terminal) coupled to a corresponding one of local bit linesLBL₁₁-LBL₃₁, respectively, a second terminal (e.g., a source/drainterminal) coupled to a corresponding one of global bit lines GBL₁-GBL₃,respectively, and a third terminal (e.g., a gate terminal) coupled torow select line SG₁.

Similarly, bit line select transistors Q₁₂-Q₃₂ are associated with localbit lines LBL₁₂-LBL₃₂, respectively, and may be used to connect localbit lines LBL₁₂-LBL₃₂ to global bit lines GBL₁-GBL₃, respectively, usingrow select line SG₂. In particular, each of bit line select transistorsQ₁₂-Q₃₂ has a first terminal (e.g., a drain/source terminal) coupled toa corresponding one of local bit lines LBL₁₂-LBL₃₂, respectively, asecond terminal (e.g., a source/drain terminal) coupled to acorresponding one of global bit lines GBL₁-GBL₃, respectively, and athird terminal (e.g., a gate terminal) coupled to row select line SG₂.

Likewise, bit line select transistors Q₁₃-Q₃₃ are associated with localbit lines LBL₁₃-LBL₃₃, respectively, and may be used to connect localbit lines LBL₁₃-LBL₃₃ to global bit lines GBL₁-GBL₃, respectively, usingrow select line SG₃. In particular, each of bit line select transistorsQ₁₃-Q₃₃ has a first terminal (e.g., a drain/source terminal) coupled toa corresponding one of local bit lines LBL₁₃-LBL₃₃, respectively, asecond terminal (e.g., a source/drain terminal) coupled to acorresponding one of global bit lines GBL₁-GBL₃, respectively, and athird terminal (e.g., a gate terminal) coupled to row select line SG₃.

Because a single bit line select transistor is associated with acorresponding local bit line, the voltage of a particular global bitline may be applied to a corresponding local bit line. Therefore, when afirst set of local bit lines (e.g., LBL₁₁-LBL₃₁) is biased to global bitlines GBL₁-GBL₃, the other local bit lines (e.g., LBL₁₂-LBL₃₂ andLBL₁₃-LBL₃₃) must either also be driven to the same global bit linesGBL₁-GBL₃ or be floated.

In one embodiment, during a memory operation, all local bit lines withinthe memory array are first biased to an unselected bit line voltage byconnecting each of the global bit lines to one or more local bit lines.After the local bit lines are biased to the unselected bit line voltage,then only a first set of local bit lines LBL₁₁-LBL₃₁ are biased to oneor more selected bit line voltages via the global bit lines GBL₁-GBL₃,while the other local bit lines (e.g., LBL₁₂-LBL₃₂ and LBL₁₃-LBL₃₃) arefloated. The one or more selected bit line voltages may correspond with,for example, one or more read voltages during a read operation or one ormore programming voltages during a programming operation.

In one embodiment, a vertical bit line memory array, such as memoryarray 400, includes a greater number of memory cells along the wordlines as compared with the number of memory cells along the vertical bitlines (e.g., the number of memory cells along a word line may be morethan 10 times the number of memory cells along a bit line). In oneexample, the number of memory cells along each bit line may be 16 or 32,whereas the number of memory cells along each word line may be 2048 ormore than 4096. Other numbers of memory cells along each bit line andalong each word line may be used.

In one embodiment of a read operation, the data stored in a selectedmemory cell (e.g., memory cell M₁₁₁) may be read by biasing the wordline connected to the selected memory cell (e.g., selected word lineWL₁₀) to a selected word line voltage in read mode (e.g., 0V). The localbit line (e.g., LBL₁₁) coupled to the selected memory cell (M₁₁₁) isbiased to a selected bit line voltage in read mode (e.g., 1 V) via theassociated bit line select transistor (e.g., Q₁₁) coupled to theselected local bit line (LBL₁₁), and the global bit line (e.g., GBL₁)coupled to the bit line select transistor (Q₁₁). A sense amplifier maythen be coupled to the selected local bit line (LBL₁₁) to determine aread current I_(READ) of the selected memory cell (M₁₁₁). The readcurrent I_(READ) is conducted by the bit line select transistor Q₁₁, andmay be between about 100 nA and about 500 nA, although other readcurrents may be used.

In one embodiment of a write operation, data may be written to aselected memory cell (e.g., memory cell M₂₂₁) by biasing the word lineconnected to the selected memory cell (e.g., WL₂₀) to a selected wordline voltage in write mode (e.g., 5V). The local bit line (e.g., LBL₂₁)coupled to the selected memory cell (M₂₂₁) is biased to a selected bitline voltage in write mode (e.g., 0 V) via the associated bit lineselect transistor (e.g., Q₂₁) coupled to the selected local bit line(LBL₂₁), and the global bit line (e.g., GBL₂) coupled to the bit lineselect transistor (Q₂₁). During a write operation, a programming currentI_(PGRM) is conducted by the associated bit line select transistor Q₂₁,and may be between about 3 uA and about 6 uA, although other programmingcurrents may be used.

FIG. 4B depicts an embodiment of a portion of a monolithicthree-dimensional memory array that includes vertical strips of anon-volatile memory material. The physical structure depicted in FIG. 4Bmay include one implementation for a portion of the monolithicthree-dimensional memory array depicted in FIG. 4A. The vertical stripsof non-volatile memory material may be formed in a direction that isperpendicular to a substrate (e.g., in the z-direction). A verticalstrip of the non-volatile memory material 414 may include, for example,a vertical oxide layer, a vertical reversible resistance-switchingmaterial (e.g., a metal oxide layer such as nickel oxide, hafnium oxide,or other similar metal oxide material, a phase change material or othersimilar reversible resistance-switching material), or a vertical chargetrapping layer (e.g., a layer of silicon nitride). The vertical strip ofmaterial may include a single continuous layer of material that may beused by a plurality of memory cells or devices.

In one example, portions of the vertical strip of the non-volatilememory material 414 may include a part of a first memory cell associatedwith the cross section between WL₁₂ and LBL₁₃ and a part of a secondmemory cell associated with the cross section between WL₂₂ and LBL₁₃. Insome cases, a vertical bit line, such as LBL₁₃, may include a verticalstructure (e.g., a rectangular prism, a cylinder, or a pillar) and thenon-volatile material may completely or partially surround the verticalstructure (e.g., a conformal layer of phase change material surroundingthe sides of the vertical structure).

As depicted, each of the vertical bit lines may be connected to one of aset of global bit lines via an associated vertically-oriented bit lineselect transistor (e.g., Q₁₁, Q₁₂, Q₁₃, Q₂₃). Each vertically-orientedbit line select transistor may include a MOS device (e.g., an NMOSdevice) or a vertical thin-film transistor (TFT).

In an embodiment, each vertically-oriented bit line select transistor isa vertically-oriented pillar-shaped TFT coupled between an associatedlocal bit line pillar and a global bit line. In an embodiment, thevertically-oriented bit line select transistors are formed in a pillarselect layer formed above a CMOS substrate, and a memory layer thatincludes multiple layers of word lines and memory elements is formedabove the pillar select layer.

FIGS. 5A-5D depict various views of an embodiment of a portion of amonolithic three-dimensional memory array 500 that includes verticalstrips of a non-volatile memory material. The physical structuredepicted in FIGS. 5A-5D may include one implementation for a portion ofthe monolithic three-dimensional memory array depicted in FIG. 4A.

Monolithic three-dimensional memory array 500 includes vertical bitlines LBL₁₁-LBL₃₃ arranged in a first direction (e.g., a z-direction),word lines WL₁₀, WL₁₁, . . . , WL₄₃ arranged in a second direction(e.g., an x-direction) perpendicular to the first direction, row selectlines SG₁, SG₂, SG₃ arranged in the second direction, and global bitlines GBL₁, GBL₂, GBL₃ arranged in a third direction (e.g., ay-direction) perpendicular to the first and second directions. Verticalbit lines LBL₁₁-LBL₃₃ are disposed above global bit lines GBL₁, GBL₂,GBL₃, which each have a long axis in the second (e.g., x-direction).Person of ordinary skill in the art will understand that monolithicthree-dimensional memory arrays, such as monolithic three-dimensionalmemory array 500 may include more or fewer than twenty-four word lines,three row select lines, three global bit lines, and nine vertical bitlines.

In an embodiment, global bit lines GBL₁, GBL₂, GBL₃ are disposed above asubstrate 502, such as a silicon, germanium, silicon-germanium, undoped,doped, bulk, silicon-on-insulator (“SOI”) or other substrate with orwithout additional circuitry. In an embodiment, an isolation layer 504,such as a layer of silicon dioxide, silicon nitride, silicon oxynitrideor any other suitable insulating layer, is formed above substrate 502.

In an embodiment, a first dielectric material layer 508 (e.g., silicondioxide) and a second dielectric material layer 510 (e.g., silicondioxide) are formed above isolation layer 504. Global bit lines GBL₁,GBL₂, GBL₃ are disposed above isolation layer 504 and are separated byfirst dielectric material layer 508. Row select lines SG₁, SG₂, SG₃ aredisposed above global bit lines GBL₁, GBL₂, GBL₃. A first etch stoplayer 512 (e.g., silicon nitride) is disposed above second dielectricmaterial layer 510. A stack of word lines WL₁₀, WL₁₁, . . . , WL₄₃ isdisposed above first etch stop layer 512, with a third dielectricmaterial layer 514 (e.g., silicon dioxide) separating adjacent wordlines. A second etch stop layer 516 (e.g., silicon nitride) is disposedabove the stack of word lines WL₁₀, WL₁₁, . . . , WL₄₃.

In an embodiment, vertical strips of a non-volatile memory material 518are disposed adjacent word lines WL₁₀, WL₁₁, . . . , WL₄₃. The verticalstrips of non-volatile memory material 518 may be formed in the firstdirection (e.g., the z-direction). A vertical strip of non-volatilememory material 518 may include, for example, a vertical oxide layer, avertical reversible resistance-switching material (e.g., a metal oxidelayer such as nickel oxide, hafnium oxide, or other similar metal oxidematerial, a phase change material or other similar reversibleresistance-switching material), a vertical charge trapping layer (e.g.,a layer of silicon nitride), or other non-volatile memory material. Avertical strip of non-volatile memory material 518 may include a singlecontinuous layer of material that may be used by a plurality of memorycells or devices.

Vertical bit lines LBL₁₁-LBL₃₃ are formed of a conductive material(e.g., a highly doped polysilicon material). Vertical bit linesLBL₁₁-LBL₃₃ also may include an adhesion layer 520 (e.g., titaniumnitride) disposed on an outer surface of each vertical bit lineLBL₁₁-LBL₃₃. In some embodiments, each of a vertical bit linesLBL₁₁-LBL₃₃ includes a vertical structure (e.g., a rectangular prism, acylinder, or a pillar), and the vertical strip of non-volatile memorymaterial 518 may completely or partially surround the vertical structure(e.g., a conformal layer of reversible resistance-switching materialsurrounding the sides of the vertical structure).

A memory cell is disposed between the intersection of each vertical bitline and each word line. For example, a memory cell M₁₁₁ is disposedbetween vertical bit line LBL₁₁ and word line WL₁₀, a memory cell M₁₁₆is disposed between vertical bit line LBL₁₃ and word line WL₁₃, a memorycell M₄₁₁ is disposed between vertical bit line LBL₁₁ and word lineWL₄₀, a memory cell M₄₃₂ is disposed between vertical bit line LBL₃₁ andword line WL₄₁, and a memory cell M₄₃₆ is disposed between vertical bitline LBL₃₃ and word line WL₄₃. In an embodiment, monolithicthree-dimensional memory array 500 includes seventy-two memory cellsM₁₁₁, M₁₁₂, . . . , M₄₃₆. Persons of ordinary skill in the art willunderstand that monolithic three-dimensional memory arrays may includemore or fewer than seventy-two memory cells.

In one example, portions of the vertical strip of non-volatile memorymaterial 518 may include a part of memory cell M₁₁₁ associated with thecross section between WL₁₀ and LBL₁₁, and a part of memory cell M₂₁₁associated with the cross section between WL₂₀ and LBL₁₁.

Each of memory cells M₁₁₁, M₁₁₂, . . . , M₄₃₆ may include a floatinggate device, a charge trap device (e.g., using a silicon nitridematerial), a resistive change memory device, or other type of memorydevice. Vertically-oriented bit line select transistors Q₁₁-Q₃₃ may beused to select a corresponding one of vertical bit lines LBL₁₁-LBL₃₃.Vertically-oriented bit line select transistors Q₁₁-Q₃₃ may be fieldeffect transistors, although other transistors types may be used.

Each of vertically-oriented bit line select transistors Q₁₁-Q₃₃ has afirst terminal (e.g., a drain/source terminal), a second terminal (e.g.,a source/drain terminal), and a third terminal (e.g., a gate terminal).The gate terminal may be disposed on all sides of thevertically-oriented bit line select transistor, and is also referred toherein as a “surround gate.” The surround gate may be used toselectively induce a conductive channel between the first terminal andthe second terminal of the transistor to turn ON and OFF thevertically-oriented bit line select transistor.

In an embodiment, the surround gates of vertically-oriented bit lineselect transistors Q₁₁, Q₂₁ and Q₃₁ are coupled to row select line SG₁,the surround gates of vertically-oriented bit line select transistorsQ₁₂, Q₂₂ and Q₃₂ are coupled to row select line SG₂, and the surroundgates of vertically-oriented bit line select transistors Q₁₃, Q₂₃ andQ₃₃ are coupled to row select line SG₃. Other configurations also may beused.

Thus, row select line SG₁ and vertically-oriented bit line selecttransistors Q₁₁, Q₂₁ and Q₃₁ may be used to select vertical bit linesLBL₁₁, LBL₂₁ and LBL₃₁, row select line SG₂ and vertically-oriented bitline select transistors Q₁₂, Q₂₂ and Q₃₂ may be used to select verticalbit lines LBL₁₂, LBL₂₂ and LBL₃₂, and row select line SG₃ andvertically-oriented bit line select transistors Q₁₃, Q₂₃ and Q₃₃ may beused to select vertical bit lines LBL₁₃, LBL₂₃ and LBL₃₃. Withoutwanting to be bound by any particular theory, for each ofvertically-oriented bit line select transistors Q₁₁-Q₃₃, it is believedthat the current drive capability of the transistor may be increased byusing a surround gate to turn ON the transistor.

Referring to FIGS. 5A and 5D, vertically-oriented bit line selecttransistors Q₁₁, Q₁₂, Q₁₃ are used to selectively connect/disconnectvertical bit lines LBL₁₁, LBL₁₂, LBL₁₃ to/from global bit line GBL₁using row select lines SG₁, SG₂, SG₃, respectively. In particular, eachof vertically-oriented bit line select transistors Q₁₁, Q₁₂, Q₁₃ has afirst terminal (e.g., a drain./source terminal) coupled to acorresponding one of vertical bit lines LBL₁₁, LBL₁₂, LBL₁₃,respectively, a second terminal (e.g., a source/drain terminal) coupledto global bit line GBL₁, and a third terminal (e.g., a surround gate)coupled to row select line SG₁, SG₂, SG₃, respectively.

Row select lines SG₁, SG₂, SG₃ are used to turn ON/OFFvertically-oriented bit line select transistors Q₁₁, Q₁₂, Q₁₃,respectively, to connect/disconnect vertical bit lines LBL₁₁, LBL₁₂,LBL₁₃, respectively, to/from global bit line GBL₁. A gate dielectricmaterial layer 522 (e.g., silicon dioxide) is disposed between rowselect lines SG₁, SG₂, SG₃ and vertically-oriented bit line selecttransistors Q₁₁, Q₁₂, Q₁₃.

Likewise, referring to FIGS. 5A-5D, vertically-oriented bit line selecttransistors Q₁₁, Q₂₁, . . . , Q₃₃ are used to selectivelyconnect/disconnect vertical bit lines LBL₁₁, LBL₂₁, LBL₃₁ to global bitlines GBL₁, GBL₂, GBL₃, respectively, using row select line SG₁. Inparticular, each of vertically-oriented bit line select transistors Q₁₁,Q₂₁, Q₃₁ has a first terminal (e.g., a drain./source terminal) coupledto a corresponding one of vertical bit lines LBL₁₁, LBL₂₁, LBL₃₁,respectively, a second terminal (e.g., a source/drain terminal) coupledto a corresponding one of global bit lines GBL₁, GBL₂, GBL₃,respectively, and a control terminal (e.g., a surround gate) coupled torow select line SG₁. Row select line SG₁ is used to turn ON/OFFvertically-oriented bit line select transistors Q₁₁, Q₂₁, Q₃₁ toconnect/disconnect vertical bit lines LBL₁₁, LBL₂₁, LBL₃₁, respectively,to/from global bit lines GBL₁, GBL₂, GBL₃, respectively.

Similarly, vertically-oriented bit line select transistors Q₁₃, Q₂₃, Q₃₃are used to selectively connect/disconnect vertical bit lines LBL₁₃,LBL₂₃, LBL₃₃ to/from global bit lines GBL₁, GBL₂, GBL₃, respectively,using row select line SG₃. In particular, each of vertically-orientedbit line select transistors Q₁₃, Q₂₃, Q₃₃ has a first terminal (e.g., adrain./source terminal) coupled to a corresponding one of vertical bitlines LBL₁₃, LBL₂₃, LBL₃₃, respectively, a second terminal (e.g., asource/drain terminal) coupled to a corresponding one of global bitlines GBL₁, GBL₂, GBL₃, respectively, and a control terminal (e.g., asurround gate) coupled to row select line SG₃. Row select line SG₃ isused to turn ON/OFF vertically-oriented bit line select transistors Q₁₃,Q₂₃, Q₃₃ to connect/disconnect vertical bit lines LBL₁₃, LBL₂₃, LBL₃₃,respectively, to/from global bit lines GBL₁, GBL₂, GBL₃, respectively.

Referring now to FIGS. 6A-6L2, an example method of forming a portion ofa monolithic three-dimensional memory array, such as monolithicthree-dimensional array 500 of FIGS. 5A-5D, is described.

With reference to FIGS. 6A-6C, substrate 502 is shown as having alreadyundergone several processing steps. Substrate 502 may be any suitablesubstrate such as a silicon, germanium, silicon-germanium, undoped,doped, bulk, silicon-on-insulator (“SOI”) or other substrate with orwithout additional circuitry. For example, substrate 502 may include oneor more n-well or p-well regions (not shown). Isolation layer 504 isformed above substrate 502. In some embodiments, isolation layer 504 maybe a layer of silicon dioxide, silicon nitride, silicon oxynitride orany other suitable insulating layer.

Following formation of isolation layer 504, a conductive material layer506 is deposited over isolation layer 504. Conductive material layer 506may include any suitable conductive material such as tungsten or anotherappropriate metal, heavily doped semiconductor material, a conductivesilicide, a conductive silicide-germanide, a conductive germanide, orthe like deposited by any suitable method (e.g., CVD, PVD, etc.). In atleast one embodiment, conductive material layer 506 may comprise betweenabout 200 and about 2500 angstroms of tungsten. Other conductivematerial layers and/or thicknesses may be used. In some embodiments, anadhesion layer (not shown), such as titanium nitride or other similaradhesion layer material, may be disposed between isolation layer 504 andconductive material layer 506, and/or between conductive material layer506 and subsequent vertically-oriented bit line select transistorslayers.

Persons of ordinary skill in the art will understand that adhesionlayers may be formed by PVD or another method on conductive materiallayers. For example, adhesion layers may be between about 20 and about500 angstroms, and in some embodiments about 100 angstroms, of titaniumnitride or another suitable adhesion layer such as tantalum nitride,tungsten nitride, tungsten, molybdenum, combinations of one or moreadhesion layers, or the like. Other adhesion layer materials and/orthicknesses may be employed.

Following formation of conductive material layer 506, conductivematerial layer 506 is patterned and etched. For example, conductivematerial layer 506 may be patterned and etched using conventionallithography techniques, with a soft or hard mask, and wet or dry etchprocessing. In at least one embodiment, conductive material layer 506 ispatterned and etched to form global bit lines GBL₁, GBL₂, GBL₃. Examplewidths for global bit lines GBL₁, GBL₂, GBL₃ and/or spacings betweenglobal bit lines GBL₁, GBL₂, GBL₃ range between about 480 angstroms andabout 1000 angstroms, although other conductor widths and/or spacingsmay be used.

After global bit lines GBL₁, GBL₂, GBL₃ have been formed, a firstdielectric material layer 508 is formed over substrate 502 to fill thevoids between global bit lines GBL₁, GBL₂, GBL₃. For example,approximately 3000-7000 angstroms of silicon dioxide may be deposited onthe substrate 502 and planarized using chemical mechanical polishing oran etchback process to form a planar surface 600. Other dielectricmaterials such as silicon nitride, silicon oxynitride, low Kdielectrics, etc., and/or other dielectric material layer thicknessesmay be used. Example low K dielectrics include carbon doped oxides,silicon carbon layers, or the like.

In other embodiments, global bit lines GBL₁, GBL₂, GBL₃ may be formedusing a damascene process in which first dielectric material layer 508is formed, patterned and etched to create openings or voids for globalbit lines GBL₁, GBL₂, GBL₃. The openings or voids then may be filledwith conductive layer 506 (and/or a conductive seed, conductive filland/or barrier layer if needed). Conductive material layer 506 then maybe planarized to form planar surface 600.

Following planarization, the semiconductor material used to formvertically-oriented bit line select transistors Q₁₁-Q₃₃ is formed overplanarized top surface 600 of substrate 502. In some embodiments, eachvertically-oriented bit line select transistor is formed from apolycrystalline semiconductor material such as polysilicon, an epitaxialgrowth silicon, a polycrystalline silicon-germanium alloy, polygermaniumor any other suitable material. Alternatively, vertically-oriented bitline select transistors Q₁₁-Q₃₃ may be formed from a wide band-gapsemiconductor material, such as ZnO, InGaZnO, or SiC, which may providea high breakdown voltage, and typically may be used to providejunctionless FETs. Persons of ordinary skill in the art will understandthat other materials may be used.

In some embodiments, each vertically-oriented bit line select transistorQ₁₁-Q₃₃ may include a first region (e.g., n+ polysilicon), a secondregion (e.g., p polysilicon) and a third region (e.g., n+ polysilicon)to form drain/source, body, and source/drain regions, respectively, of avertical FET. For example, a heavily doped n+ polysilicon layer 602 maybe deposited on planarized top surface 600. In some embodiments, n+polysilicon layer 602 is in an amorphous state as deposited. In otherembodiments, n+ polysilicon layer 602 is in a polycrystalline state asdeposited. CVD or another suitable process may be employed to deposit n+polysilicon layer 602.

In an embodiment, n+ polysilicon layer 602 may be formed, for example,from about 100 to about 500 angstroms, of phosphorus or arsenic dopedsilicon having a doping concentration of about 10²¹ cm⁻³. Other layerthicknesses, doping types and/or doping concentrations may be used. N+polysilicon layer 602 may be doped in situ, for example, by flowing adonor gas during deposition. Other doping methods may be used (e.g.,implantation).

After deposition of n+ silicon layer 602, a doped p-type silicon layer604 may be formed over n+ polysilicon layer 602. P-type silicon may beeither deposited and doped by ion implantation or may be doped in situduring deposition to form a p-type silicon layer 604. For example, anintrinsic silicon layer may be deposited on n+ polysilicon layer 602,and a blanket p-type implant may be employed to implant boron apredetermined depth within the intrinsic silicon layer. Exampleimplantable molecular ions include BF₂, BF₃, B and the like. In someembodiments, an implant dose of about 1-10×10¹³ ions/cm² may beemployed. Other implant species and/or doses may be used. Further, insome embodiments, a diffusion process may be employed. In an embodiment,the resultant p-type silicon layer 604 has a thickness of from about 800to about 4000 angstroms, although other p-type silicon layer sizes maybe used.

Following formation of p-type silicon layer 604, a heavily doped n+polysilicon layer 606 is deposited on p-type silicon layer 604. In someembodiments, n+ polysilicon layer 606 is in an amorphous state asdeposited. In other embodiments, n+ polysilicon layer 606 is in apolycrystalline state as deposited. CVD or another suitable process maybe employed to deposit n+ polysilicon layer 606.

In an embodiment, n+ polysilicon layer 606 may be formed, for example,from about 100 to about 500 angstroms of phosphorus or arsenic dopedsilicon having a doping concentration of about 10²¹ cm⁻³. Other layerthicknesses, doping types and/or doping concentrations may be used. N+polysilicon layer 606 may be doped in situ, for example, by flowing adonor gas during deposition. Other doping methods may be used (e.g.,implantation). Persons of ordinary skill in the art will understand thatsilicon layers 602, 604 and 606 alternatively may be doped p+/n/p+,respectively, or may be doped with a single type of dopant to producejunctionless-FETs.

Following formation of n+ polysilicon layer 606, a hard mask materiallayer 608 is deposited on n+ polysilicon layer 606, resulting in thestructure shown in FIGS. 6B1-6B3. Hard mask material layer 608 mayinclude any suitable hard mask material such as silicon nitride,amorphous carbon, or the like deposited by any suitable method (e.g.,CVD, PVD, etc.). In at least one embodiment, hard mask material layer608 may comprise between about 300 angstroms and about 800 angstroms ofsilicon nitride. Other hard mask materials and/or thicknesses may beused.

Hard mask material layer 608 and silicon layers 602, 604 and 606 arepatterned and etched to form rows 610 of silicon layers 602, 604 and 606and hard mask material layer 608, resulting in the structure shown inFIGS. 6C1-6C3. For example, hard mask material layer 608 and siliconlayers 602, 604 and 606 may be patterned and etched using conventionallithography techniques, with wet or dry etch processing.

Hard mask material layer 608 and silicon layers 602, 604 and 606 may bepatterned and etched in a single pattern/etch procedure or usingseparate pattern/etch steps. Any suitable masking and etching processmay be used to form vertical transistor pillars. For example, siliconlayers may be patterned with about 1 to about 1.5 micron, morepreferably about 1.2 to about 1.4 micron, of photoresist (“PR”) usingstandard photolithographic techniques. Thinner PR layers may be usedwith smaller critical dimensions and technology nodes. In someembodiments, an oxide hard mask may be used below the PR layer toimprove pattern transfer and protect underlying layers during etching.

In some embodiments, after etching, rows 610 may be cleaned using adilute hydrofluoric/sulfuric acid clean. Such cleaning may be performedin any suitable cleaning tool, such as a Raider tool, available fromSemitool of Kalispell, Mont. Example post-etch cleaning may includeusing ultra-dilute sulfuric acid (e.g., about 1.5 1.8 wt %) for about 60seconds and/or ultra-dilute hydrofluoric (“HF”) acid (e.g., about0.4-0.6 wt %) for 60 seconds. Megasonics may or may not be used. Otherclean chemistries, times and/or techniques may be employed.

A first gate dielectric material layer 522 a is deposited conformallyover substrate 502, and forms on sidewalls of rows 610. For example,between about 30 angstroms to about 100 angstroms of silicon dioxide maybe deposited. Other dielectric materials such as silicon nitride,silicon oxynitride, low K dielectrics, etc., and/or other dielectricmaterial layer thicknesses may be used.

An anisotropic etch is used to remove lateral portions of first gatedielectric material, leaving only sidewall portions of first gatedielectric material layer 522 a, resulting in the structure shown inFIGS. 6D1-6D3.

A first gate electrode material is deposited over rows 610 and firstgate dielectric material layer 522 a. For example, approximately 50angstroms to about 500 angstroms of a highly-doped semiconductor, suchas n+ polysilicon, p+ polysilicon, or other similar conductive materialmay be deposited. Other conductive materials and/or thicknesses may beused. The as-deposited gate electrode material is subsequently etchedback to form first rail gates 612 a, resulting in the structure shown inFIGS. 6E1-6E3. First rail gate 612 a are disposed on first sides of thevertical stack of silicon layers 602, 604 and 606. Other conductivematerials and/or thicknesses may be used for first rail gates 612 a.

A first liner material layer 614 is deposited conformally over substrate502, hand forms on sidewalls of rows 610. For example, between about 30angstroms to about 100 angstroms of silicon dioxide may be deposited.Other dielectric materials such as silicon nitride, silicon oxynitride,low K dielectrics, etc., and/or other dielectric material layerthicknesses may be used. An anisotropic etch is used to remove lateralportions of liner material, leaving only sidewall portions of firstliner material layer 614.

A sacrificial material layer 616 is deposited over substrate 602,filling voids between first liner material layers 614. For example,approximately 200 angstroms to about 500 angstroms of polysilicon 616may be deposited and planarized using chemical mechanical polishing oran etch-back process to form planar top surface 618, resulting in thestructure shown in FIGS. 6F1-6F4. Other sacrificial materials andthicknesses may be used.

Following formation of sacrificial material layer 616, sacrificialmaterial layer 616, first liner material layers 614, first rail gates612 a, hard mask material layer 608, first gate dielectric materiallayer 522 a, and silicon layers 602, 604 and 606 are patterned andetched to form vertical transistor pillars, resulting in the structureshown in FIGS. 6G1-6G4. For example, sacrificial material layer 616,sacrificial material layer 616, first liner material layers 614, hardmask material layer 608, first rail gates 612 a, first gate dielectricmaterial layer 522 a, and silicon layers 602, 604 and 606 may bepatterned and etched using conventional lithography techniques, with wetor dry etch processing.

In an embodiment, sacrificial material layer 616, first liner materiallayers 614, first rail gates 612 a, hard mask material layer 608, firstgate dielectric material layer 522 a, and silicon layers 602, 604 and606 are patterned and etched to form vertical transistor pillarsdisposed above global bit lines GBL₁, GBL₂, GBL₃. The verticaltransistor pillars will be used to form vertically-oriented bit lineselect transistors Q₁₁-Q₃₃.

A second gate dielectric material layer 522 b is deposited conformallyover substrate 502, and forms on sidewalls of silicon layers 602, 604and 606. For example, between about 30 angstroms to about 100 angstromsof silicon dioxide may be deposited. Other dielectric materials such assilicon nitride, silicon oxynitride, low K dielectrics, etc., and/orother dielectric material layer thicknesses may be used.

An anisotropic etch is used to remove lateral portions of second gatedielectric material, leaving only sidewall portions of second gatedielectric material layer 522 b, resulting in the structure shown inFIGS. 6H1-6H4.

A second gate electrode material is deposited over second gatedielectric material layer 522 b. For example, approximately 50 angstromsto about 500 angstroms of a highly-doped semiconductor, such as n+polysilicon, p+ polysilicon, or other similar conductive material may bedeposited. The as-deposited gate electrode material is subsequentlyetched back to form second rail gates 612 b, resulting in the structureshown in FIGS. 6I1-6I4. Second rail gate 612 b are disposed on secondsides of the vertical stack of silicon layers 602, 604 and 606. Otherconductive materials and/or thicknesses may be used for second railgates 612 b. In addition, first rail gates 612 a and second rail gates612 b may be the same material or may be different materials.

A second liner material layer 620 is deposited conformally oversubstrate 502, and forms on sidewalls of second rail gates 612 b. Forexample, between about 30 angstroms to about 100 angstroms of siliconnitride may be deposited. Other dielectric materials such as silicondioxide, silicon oxynitride, low K dielectrics, etc., and/or otherdielectric material layer thicknesses may be used. An anisotropic etchis used to remove lateral portions of liner material, leaving onlysidewall portions of second liner material layer 620.

A second dielectric material layer 510 is deposited over substrate 502,filling voids between second liner material layer 620. For example,approximately 5000 to about 8000 angstroms of silicon dioxide may bedeposited and planarized using chemical mechanical polishing or anetch-back process to form planar top surface 622, resulting in thestructure shown in FIGS. 6J1-6J2.

Next, first liner material layers 614, sacrificial material layers 616and portions of second gate dielectric material layers 522 b adjacent tosacrificial material layers 616 are removed, for example by a wet or dryetch process, resulting in the structure shown in FIGS. 6K1-6K2.

A third gate electrode material is deposited over substrate 502. Forexample, approximately 50 angstroms to about 1000 angstroms of ahighly-doped semiconductor, such as n+ polysilicon, p+ polysilicon, orother similar conductive material may be deposited. The as-depositedgate electrode material is subsequently etched back to the same heightas first rail gates 612 a and second rail gates 612 b to form bridgesections 612 c, resulting in the structure shown in FIGS. 6L1-6L2. Otherconductive materials and/or thicknesses may be used for bridge sections612 c. First rail gates 612 a, second rail gates 612 b and bridgesections 612 c collectively form row select lines SG₁, SG₂, SG₃. Firstrail gates 612 a, second rail gates 612 b and bridge sections 612 c maybe the same material or may be different materials.

Thus, the example process steps described above and illustrated in FIGS.6A1-6L2 form vertically-oriented bit line select transistors Q₁₁-Q₃₃that include surround gates formed by first rail gates 612 a, secondrail gates 612 b and bridge sections 612 c coupled between first railgates 612 a and second rail gates 612 b, and disposed on all sides ofthe vertically-oriented bit line select transistors.

In particular, the surround gates of vertically-oriented bit line selecttransistors Q₁₁, Q₂₁ and Q₃₁ are coupled to row select line SG₁, thesurround gates of vertically-oriented bit line select transistors Q₁₂,Q₂₂ and Q₃₂ are coupled to row select line SG₂, and the surround gatesof vertically-oriented bit line select transistors Q₁₃, Q₂₃ and Q₃₃ arecoupled to row select line SG₃.

Following the example process illustrated in FIGS. 6A1-6L2, additionalprocess steps may be used to form word lines WL₁₀, WL₁₁, . . . , WL₄₃arranged and vertical bit lines LBL₁₁-LBL₃₃ above vertically-orientedbit line select transistors Q₁₁-Q₃₃ and global bit lines GBL₁-GBL₃.

Thus, as described above, one embodiment of the disclosed technologyincludes a method that includes forming a transistor by forming a firsta rail gate disposed in a first direction above a substrate, forming asecond rail gate disposed in a second direction above the substrate, thesecond direction perpendicular to the first direction, and forming abridge section disposed between the first rail gate and the second railgate.

One embodiment of the disclosed technology includes an apparatus thatincludes a monolithic three-dimensional memory array that has avertically-oriented bit line select transistor including a gateterminal. The gate terminal includes a first a rail gate disposed in afirst direction above a substrate, a second rail gate disposed in asecond direction above the substrate, the second direction perpendicularto the first direction, and a bridge section disposed between the firstrail gate and the second rail gate.

One embodiment of the disclosed technology includes a method thatincludes forming a vertically-oriented bit line select transistorincluding a stack of semiconductor material above a substrate, forming afirst a rail gate disposed on a first side of the stack of semiconductormaterial, forming a second rail gate disposed on a second side of thestack of semiconductor material, the second side substantiallyperpendicular to the first side, and forming a bridge section disposedbetween the first rail gate and the second rail gate.

For purposes of this document, each process associated with thedisclosed technology may be performed continuously and by one or morecomputing devices. Each step in a process may be performed by the sameor different computing devices as those used in other steps, and eachstep need not necessarily be performed by a single computing device.

For purposes of this document, reference in the specification to “anembodiment,” “one embodiment,” “some embodiments,” or “anotherembodiment” may be used to described different embodiments and do notnecessarily refer to the same embodiment.

For purposes of this document, a connection can be a direct connectionor an indirect connection (e.g., via another part).

For purposes of this document, the term “set” of objects may refer to a“set” of one or more of the objects.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

The invention claimed is:
 1. A method comprising: forming a transistorby: forming a first a rail gate disposed in a first direction above asubstrate; forming a second rail gate disposed in a second directionabove the substrate, the second direction perpendicular to the firstdirection; and forming a bridge section disposed between the first railgate and the second rail gate.
 2. The method of claim 1, wherein forminga transistor further comprises: forming a vertical stack ofsemiconductor material; forming the first rail gate on first sides ofthe vertical stack of semiconductor material; and forming the secondrail gate on second sides of the vertical stack of semiconductormaterial.
 3. The method of claim 2, wherein forming a transistor furthercomprises forming a dielectric material between the first rail gate andthe first sides of the vertical stack of semiconductor material, andbetween the second rail gate and the second sides of the vertical stackof semiconductor material.
 4. The method of claim 2, wherein forming avertical stack of semiconductor material comprises forming a firstregion having a first conductivity type, a second region having a secondconductivity type and a third region having the first conductivity type.5. The method of claim 1, wherein the transistor comprises avertically-oriented field effect transistor.
 6. The method of claim 1,wherein the first rail gate, the second rail gate and the bridge sectioncomprise a surround gate of the transistor.
 7. The method of claim 1,wherein the transistor comprises a first terminal, a second terminal anda third terminal, and the method further comprises coupling the firstterminal to a vertical bit line of a memory array, coupling the secondterminal to a global bit line of a memory array, and coupling the thirdterminal to the first rail gate, the second rail gate and the bridgesection.
 8. The method of claim 7, wherein memory array comprises amemory cell comprising a vertical strip of non-volatile memory material.9. The method of claim 8, wherein the nonvolatile memory materialcomprises any of a reversible resistance-switching material, a phasechange material, and a charge trapping layer.
 10. The method of claim 8,wherein the nonvolatile memory material comprises one or more of HfO₂,Al₂O₃, HfSiO_(x), HfSiO_(x)N_(y), HfAlO_(x), Nb₂O₅, Ta₂O₅, ZrO₂, Cr₂O₃,Fe₂O₃, Ni₂O₃, Co₂O₃, WO₃, TiO₂, SrZrO₃, SrTiO₃.
 11. An apparatuscomprising: a monolithic three-dimensional memory array comprising avertically-oriented bit line select transistor comprising a gateterminal comprising: a first a rail gate disposed in a first directionabove a substrate; a second rail gate disposed in a second directionabove the substrate, the second direction perpendicular to the firstdirection; and a bridge section disposed between the first rail gate andthe second rail gate.
 12. The apparatus of claim 11, wherein thevertically-oriented bit line select transistor comprises a surround gatecomprising the first rail gate, the second rail gate and the bridgesection.
 13. The apparatus of claim 11, wherein the first rail gate, thesecond rail gate and the bridge section each comprise highly dopedsemiconductor material.
 14. The apparatus of claim 11, wherein thevertically-oriented bit line select transistor comprises a firstterminal coupled to a vertical bit line of the monolithicthree-dimensional memory array, a second terminal coupled to a globalbit line of the monolithic three-dimensional memory array, and a thethird terminal coupled to a row select line of the monolithicthree-dimensional memory array.
 15. The apparatus of claim 11, whereinmonolithic three-dimensional memory array comprises a vertical strip ofnon-volatile memory material.
 16. The apparatus of claim 15, wherein thenonvolatile memory material comprises any of a reversibleresistance-switching material, a phase change material, and a chargetrapping layer.
 17. The apparatus of claim 15, wherein the nonvolatilememory material comprises one or more of HfO₂, Al₂O₃, HfSiO_(x),HfSiO_(x)N_(y), HfAlO_(x), Nb₂O₅, Ta₂O₅, ZrO₂, Cr₂O₃, Fe₂O₃, Ni₂O₃,Co₂O₃, WO₃, TiO₂, SrZrO₃, SrTiO₃.
 18. A method comprising: forming avertically-oriented bit line select transistor comprising a stack ofsemiconductor material above a substrate; forming a first a rail gatedisposed on a first side of the stack of semiconductor material; forminga second rail gate disposed on a second side of the stack ofsemiconductor material, the second side substantially perpendicular tothe first side; and forming a bridge section disposed between the firstrail gate and the second rail gate.
 19. The method of claim 18, whereinthe first rail gate, the second rail gate and the bridge sectioncomprise a same material.
 20. The method of claim 18, wherein the firstrail gate, the second rail gate and the bridge section comprisedifferent materials.